HARBIN - Three people have been trapped and four others are still missing after an explosion early Saturday ripped through a residential building in Northeast China's Heilongjiang province, local authorities said.

The blast occurred at 4:50 am at the sixth floor of a building in the city of Mudanjiang. Rescuers pulled three injured people from the damaged building, but found three are still trapped on the fifth floor and four more missing, according to the city's publicity department.

A probe into the cause of the blast is underway.